Field of the Invention
The present invention relates to an imaging optical system and an imaging apparatus having the same, and is suitable, for example, for a video camera, a digital still camera, a television (TV) camera, and a monitoring camera, each of which uses an image sensor, and a film camera using a silver-halide film.
Description of the Related Art
There is known a telephoto-type imaging optical system (telephoto lens) including, in order from an object side to an image side, a front lens unit having a positive refractive power and a rear lens unit having a negative refractive power. In the telephoto lens, a large amount of chromatic aberration, particularly axial chromatic aberration and lateral chromatic aberration, occurs among a variety of aberrations as the focal length becomes longer.
U.S. Pat. No. 5,757,555 discusses a telephoto lens in which chromatic aberrations are corrected by combining a positive lens using a low dispersion material having an anomalous partial dispersion, such as fluorite or product name S-FPL51 (Ohara Inc.), with a negative lens using a high dispersion material. Further, in a telephoto lens having a small F-number, large amounts of spherical aberration and comatic aberration (coma) occur among a variety of aberrations as the F-number becomes smaller. To correct the spherical aberration and coma in a telephoto lens having a small F-number, U.S. Pat. No. 5,745,306 discusses a telephoto lens having an increased number of lenses so that the degree of freedom to the aberration corrections is increased.
Meanwhile, as a method for achieving an imaging optical system in which a variety of aberrations including the chromatic aberrations of the optical system is corrected and, at the same time, the overall lens length is shortened, there is known a method using an aspheric surface lens, an optical element having an anomalous dispersion, and a diffractive optical element having a diffraction action. With the aforementioned method, there has been provided an imaging optical system in which the chromatic aberrations and mono chromatic aberrations are favorably corrected, and at the same time, overall lens length is shortened by reduction of the number of lenses. In U.S. Pat. No. 8,638,506, the refractive power of the first lens unit is increased so that the overall lens length is shortened, and at the same time mono chromatic aberration corrections are performed in an aspheric surface and chromatic aberrations are corrected by the diffractive optical element and the optical element having an anomalous dispersion.
In many imaging optical systems, focusing from an object located at a faraway distance (infinite-distance object) to an object located at a nearby distance (near-distance object) is performed by movement of the whole or a part of the imaging optical system in an optical axis direction. In the telephoto lens, an inner focus-type focusing in which a lens unit that is relatively small and light and located at a central portion in the imaging optical system is moved is often used. The telephoto lens discussed in the above-described patent documents includes, in order from the object side, a first lens unit having a positive refractive power and a second lens unit having a negative refractive power. The second lens unit moves in focusing.
In a telephoto lens, the size of the front lens unit having a positive refractive power generally becomes larger as the focal length becomes longer. It is therefore important for the telephoto lens that a lens configuration of the front lens unit is appropriately set in order to realize downsizing and weight reduction of the lens system, while at the same time, chromatic aberrations are favorably corrected, so that a telephoto lens with high optical performance is obtained. However, if the overall lens length is substantially shortened by setting a configuration of front lens unit, correction of the axial chromatic aberration and the lateral chromatic aberration in a well-balanced manner becomes difficult. Therefore, for example, correction of chromatic aberrations becomes necessary also in a rear lens unit positioned closer to the image side than an aperture stop.